(1) Field of the Invention
The invention relates to a compound helicopter with the features of the preamble of claim 1.
Typical compound helicopters comprise a fuselage, a main rotor providing lift and a pair of additional propulsion devices providing thrust and a fixed wing structure on each side in an essentially horizontal plane of the fuselage. Said fixed wing structure provides additional lift during horizontal cruise flight. The propulsion devices are arranged on said wings.
(2) Description of Related Art
The dominant helicopter configuration in the present time is based on Sikorsky's basic design with a main rotor and an auxiliary tail rotor to counter torque. Said conventional helicopters show excellent hover capabilities but suffer from limitations in terms of horizontal flight speed. These limitations are associated to two aerodynamic phenomena at the main rotor: the retreating blade stall and the maximum blade tip velocity. In general terms: The lift and thrust force capabilities of a helicopter rotor decrease with increasing forward speed.
The compound helicopters and so-called convertiplanes are basically the most relevant concepts aiming to overcome the horizontal flight deficiencies of the dominant helicopters by introducing attributes of fixed-wing aircrafts as compromise. However, a compromise between both aircraft types has always to be conveniently adapted to the planned mission profile of the helicopter.
Compound helicopters with lift compounding, thrust compounding or a combination of both basically aim to off-load the main rotor from its simultaneous lifting and propulsive duties to allow for higher forward speeds of the compound helicopter.
A lift compounding entails adding wings to a helicopter hence enabling to increase the load factor of the helicopter and to reach a higher manoeuvrability. This improves the efficiency of the helicopter at moderately high speed but at the expense of reduced efficiencies at lower forward speeds and in the hover.
A thrust compounding implies the addition of essentially horizontally oriented auxiliary propulsion units to the helicopter. This has been typically accomplished by means of a single or a pair of propellers being driven by drive shafts powered by the main turboshaft engines. The use of a pair of propulsion units has the advantage of providing for anti-torque capabilities without the need of an additional tail rotor, hence relativizing the inherent system complexity of the thrust compound configuration.
A more extended configuration of a compound helicopter includes both the addition of wings and propulsion units. The lift during cruise is simultaneously provided by the main rotor—in powered condition—usually addressed as “hybrid helicopter”—or in autorotation—“autogyro”-modus—and wings. The higher forward speed is provided by the horizontally oriented auxiliary propulsion units of the compound helicopter. The compound helicopter hence overcomes the rotor lift limits by means of the wings and the rotor thrust limits by means of the propulsion units. As a result, the benefit of a higher load factor is obtained along with potential for higher speed. The use of a pair of thrust propulsion units—opposed and both offset relative to each other and to a longitudinal axis of the helicopter—enables for a simultaneous torque correction.
Compound helicopters with two wing-mounted propellers are described in U.S. Pat. No. 3,105,659 A, US 2009/0321554 A1 and U.S. Pat. No. 6,513,752 B2. A compound helicopter with a propulsion device as a single nose-mounted propeller is disclosed in US/2005/0151001 A1 or with a single rear-mounted propeller in U.S. Pat. No. 3,241,791 A, CA 2316418 and in U.S. Pat. No. 3,448,946. Said typical configurations feature a pair of main wings located below the main rotor of the compound helicopter.
A canard configuration of a compound helicopter, featuring two main wings behind the main rotor and two nose-mounted canard wings, is described in US 2010/0065677 A1, said configuration outstanding by the lack of a tail boom and the rear position of propulsion propellers, which results in comfort by reducing noise and vibrations and increased passenger safety. Moreover, the canard configuration provides for additional lift, which means that the center of lift of the main wings is allowed to not coincide with the main rotor mast, hence allowing locating the main wings far in the rear of the helicopter. The document US 2011 0114798 A1 discloses a compound helicopter based on a tandem wing configuration with an arrangement of a pair of wings on both the fuselage front end and aft end, the propulsion devices being arranged near the tips of the front wings.
Canard configurations are—as well—not unusual in fixed-wing aircraft design. The canards can have a lifting or a control function. Lifting canards contribute to the total lift of the aircraft, hence allowing for a significant aft position of the main wings and the center of gravity (CofG). To ensure pitch stability, the lift slope of the canard wing has to be lower than that one of the main wing. Typical configurations with aft-tails have a loss of efficiency as a result of the downward lift that must be compensated with extra lift of the main wings. An important penalty arises from the aerodynamic interference between the canards and the main wings. From a structural point of view, lifting canard configurations or even tandem wing configurations offer considerable advantages.
The propulsion devices of compound helicopters are typically attached to the fuselage or arranged at the wings or at the aft end of either the tail boom or the fuselage. The attachment of the propulsion devices to the fuselage is only feasible if using turbojets, allowing undisturbed wings but not allowing anti-torque capabilities, hence still requiring an additional tail rotor. The same applies for configurations with a single main rotor and a tail-boom mounted pusher propeller. Hence propellers are typically arranged on the wings, somewhere between the wing tip and the fuselage or at the wing tip.
Typically, the winged compound helicopter features a monoplane design with one set of wing surfaces in cantilever design as either low-wing, mid-wing or shoulder-wing arrangement. A cantilever wing design has the disadvantage of requiring a central wing box carrying the wing bending moments. The central wing box increases the front masking and the associated drag in the case of a low-wing or shoulder wing design. For a mid-wing design, the central wing box disturbs an inner cabin compartment of the fuselage. For a shoulder-wing design, the main rotor moves further upwards and the aerodynamic efficiency of the wing is negatively affected by its arrangement close to the main rotor. Monoplane low-wing concepts with wing-mounted propellers are not suitable considering the requirement of the propellers being driven by the engines which are typically allocated on top of the helicopter close to the main rotor.
For existing compound helicopters with side mounted, unducted propellers, the location of propellers in the cabin area goes along with serious penalties in terms of passenger safety. In case of a blade separation, the blade has to be retained by additional structural features to avoid a cabin penetration hence additionally increasing the structural weight. Open rotors being placed close to the doors, especially in the case of using tractor propellers at the leading edge of the wings, represent further serious safety penalties during boarding and increase the noise exposure in the cabin.
The document WO 2008/085195 A discloses rotorcraft wings with improved aerodynamic efficiency by a large wing aspect ratio. However, large wing aspect ratios are disadvantageous for a monoplane compound rotorcraft with wing-tip mounted propulsion devices in terms of structural efficiency.
The documents DE 694 30 198 T2 and EP 2 418 148 A2 disclose a fixed wing aircraft design with a stiff and efficient wing configuration even for large-aspect ratios. The so-called joined-wing of box-wing configurations are characterized by the arrangement of a pair of main wings at each side of the aircraft which are interconnected at their tips. Typically, one wing is swept forward and the other is swept back.
The use of external struts for wing bracing is an effective means to reduce the loading of the wing structure, especially at its root, enabling a lighter and stiffer wing design but at the expense of increasing the aerodynamic drag to a certain extent. Advanced braced wing designs aim less noise, cleaner exhaust and lower fuel consumption.
The document GB 895590 A discloses an aircraft for vertical take-off and landing with a rotor, two wings, two or more airscrews whose pitch is reversible and independently variable, four or more interconnected power units, a fuselage having a tail unit, and means controllable by the pilot whereby power can be transmitted wholly or partially to rotor or airscrews depending upon whether the aircraft is to perform vertical or translatory flight. Turbines, located in the upper part of the fuselage, and turbines, on the wings, are connected to the rotor and airscrews by gearing and shafting, which include spur or bevel wheels and connecting shafts and speed reducing and uncoupling means. Each wing has an end portion, which can be folded downwards to lessen the influence of the wings on the lift of the rotor. Fuel tanks at the outer ends of wing portions act as side floats when the aircraft lands or rests on water. In vertical flight, the rotor torque is balanced by operation of the airscrews, one having reverse pitch relative to the other.
The document U.S. Pat. No. 5,046,684 A discloses a tiltrotor aircraft with a joined-wing configuration that eliminates some major speed-limiting constraints of prior tiltrotor configurations-thereby allowing operation into the intermediate speed range of roughly 350 to 450 knots. Joined wings offer relatively rigid, stiffened support for the additional wing-mounted hardware and also stiffen the system to resist rotor flutter and other sources of aggravated loading, that are characteristic of tiltrotor aircraft.